In the process of electrostatographic reproduction, a light image of an original to be copied or printed is typically recorded in the form of a latent electrostatic image upon a photosensitive member, with a subsequent rendering of the latent image visible by the application of electroscopic marking particles commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support medium, such as a sheet of plain paper. To render this toner image permanent, the image must be “fixed” or “fused” to the paper, generally by the application of heat and pressure.
With the advent of high speed monochrome and color marking machines, including xerography reproduction machines wherein copiers or printers can produce at a rate in excess of three thousand copies per hour, the need for improved developer performance is evident and useful.
A common goal in the design and development of electrostatographic printing devices is the ability to maintain optimum image quality from page to page and job to job regardless of the characteristics of the images being formed on each page. As should be appreciated, to maintain optimum image quality it is important that the printing device sustain good development as well as good transfer efficiency. Good development or good developability, refers to the ability of the device to transfer the appropriate amount of toner to the latent image when forming the toner powder image. Good transfer efficiency refers to the ability of the printing device to transfer the toner powder image to the substrate.
It is known that maintaining the state of the material in the developer housing within an optimum range improves developability and transfer efficiency. To accomplish this, many printing systems use a variety of processes to maintain the state of the developer materials within the optimum range by monitoring and controlling one or more characteristics of the materials including, for example, temperature, humidity, charge, toner concentration (ratio of toner to carrier) and toner charge distribution.
However, even if the developer materials are maintained in an optimal state, it has been observed that under certain conditions such as extended running of prints with low toner area coverage in one or more of the color separations, the developability and/or transfer efficiency can degrade due to changes in the materials state in the developer housing. A second such condition is running prints at any toner area coverage or range of area coverages for a prolonged period of time such that the average residence time of the carrier in the Xerographic housing surpasses some threshold value as measured in kiloprints (thousands of prints). This degradation in developability and/or transfer efficiency produces weak, mottled and/or streaky images even with the enablement of Xerographic process controls system using all of the printer's operating space to optimize the image quality of the prints. With existing printing devices, when running low area coverage prints or prints at any area coverage where the average residence time of the carrier in the Xerographic housing has surpassed some threshold value and a reduced image quality suspected to result from a degradation in developability or transfer efficiency is observed, it is known to address the problem by either changing the materials within the developer housing(s) or by running a large number of prints (e.g., 1 to 2 thousand) of a high area coverage document to remove “bad” toner from the developer housings.
Although replacing the materials within developer housing and/or running a large number of a high area coverage document can improve the developability and transfer efficiency and thus restore image quality, such procedures are both costly and time consuming as the user may be forced to interrupt the job and perform some service action on the printer. Additionally, the above processes can result in a substantial waste of toner, carrier and/or paper resources. Furthermore, as the problem must first be identified and diagnosed by an operator before any corrective action can be taken, there is the possibility of a substantial loss in productivity resulting from the loss of a large number of pages before detection of a problem or from dedicating an operator to monitor the job to detect potential problems.
In electrostatic development processes, a developer material is used comprising relatively large carrier beads that have fine toner particles electrostatically coated thereon. Various known means are used to convey these toner particles to the latent electrostatic image on the photoconductive surface. The composition of the carrier particles is so chosen as to electrostatically attract and hold the toner particles for transfer to the latent image. As the developer is directly or indirectly contacted with this photoconductor surface, the toner particles are electrostatically deposited and secured to the charged portion of this latent image and not deposited on the uncharged or background portion of the image. The carrier and excess toner are then recycled for later use but eventually after extended use become ready to be removed from the system to be replenished with new toner and carrier.
The root cause of the development (and transfer) degradation is well known; a loss of additive functionality on the toners as the average residence time of the toner increases in the housing during low AC printing. While this issue is mitigated to a large extent by design of the Xerographic toners, there exists a need to further address the problem by design of the printing engines themselves. Currently, customers may need to maintain toner throughput in a Xerographic housing at higher levels than that required to print the image content of customer jobs, for example by running high area coverage prints, developing toner to unimaged areas of the photoconductor which moves directly to the cleaning subsystem, (either when the printer is being used to run customer jobs or when it is removed from printing those jobs) and/or by changing developers when the image quality visually degrades or a system actuator, for example a development voltage reaches a critical value. (See commonly-owned U.S. Pat. No. 7,079,794 B2, the disclosure which is incorporated herein by reference.) There are substantial disadvantages to this strategy in customer flexibility, waste and downtime, and the need for a better solution is universally recognized. Introducing only fresh toner through an existing replenisher dispensing device without a corresponding withdrawal of toner from the developer housing (for example, by the mechanisms discussed above) is not a satisfactory option as such a process changes (increases) the toner concentration (TC) in a low throughput mode, quickly pushing the machine above the high TC operating boundary and bringing on a different set of image quality failures. A better way to introduce fresh toner and developer with the requisite better developability and transfer efficiency is provided by embodiments of this invention.